How to Finalize a Measurement Screw With a Bolt Strain Gauge
We know that it is difficult to bond a strain gauge into a bolt that measures the tightening stress. Normally, this requires the use of heavy machinery. However, by using special electrical strain gauges, a simplified solution can be provided for measuring the axial load on bolt elements. The installation of this gauge differs from the conventional procedures of installing a flat strain gauge. The finalization after bonding the gauge into the bolt in order to have a ready-to-use measurement screw is shown below.
This article provides instructions for how to finalize a measurement screw with HBM's 1-LB11-3/120ZW strain gauge that is already integrated in a bolt. This new gauge enables measurement by being embedded into a hole drilled through the head of the bolt.
LB11 / TB21 - Cylindrical Strain Gauges
LB11 and TB21 are cylindrical strain gauges to measure strain, force and vibration in screws, bolts, or other construction elements.
EP70 Strain Gauge Adhesive
The two-component adhesive EP70 is the first choice for the installation of cylindrical strain gauges (LB11 and TB21) in bolts and screws for strain, force and vibration measurements. The EP70 is particularly characterized by its easy and process-safe handling: Bubble formation during curing and air inclusions are no longer a problem.
SG250 Silicon Rubber Covering Material for Sensors | Tube
SG250 covering material is a transparent silicone rubber that is applied from the tube. It cures in contact with air at room temperature.
Other equipment: Lightly shielded PFA measurement cable (order no: 1-CABP4/20), LS7 solder pads (order no: 1-LS7), connector for MX1615b/MX1616B modules (order no: 1-CON-S1015)
Video - How to Bond a Strain Gauge into a Bolt
Watch this video and learn how to built up a measurement screw by integrating the cylindric strain gauge LB11 and how to wire it to a connector ready-to-use- with a bridge amplifier module.
Wiring of a Bolt Strain Gauge
1. We begin with an already-bonded LB11 strain gauge integrated with a bolt. The strain gauge is fixed in the screw and is equipped with 60 mm copper wires. For further information on how to bond this strain gauge into a bolt, refer to the information available on the HBM website.
2. In the next step, the copper wires of the screw are to be connected to the solder terminals. In this case, we position the solder terminals on the side of the screw head. The solder terminals can be bonded, for example, with HBM's EP70 hot-curing epoxy resin adhesive. Then, we use a rubber pen to eliminate any oxidations on the surface of the solder terminal.
3. Thereafter, we use a soldering iron to pre-tin the solder pad. This makes it easier to solder the copper wire of the gauge later.
4. In the next step, the two copper wires of the strain gauge are connected with the solder terminal.
Tip: We recommend guiding the copper wires close to the screw and minimizing the free length in order to avoid damaging the strain gauge. Use a tweezer to carefully handle the copper wires.
The copper wires are paint-insulated as well as flexible and can be bent around any object with a small radius.
5. Now, the measurement wire needs to be installed between the DAQ system and the gauge. Therefore, we use a 4-wire cable from the HBM portfolio (1-CABP4/20).
6. The screw is connected with a push-in connector, which is suitable for the MX1615B and MX1616B modules.
The wiring of the finalized measurement screw looks as follows. The two paint-insulated copper wires are soldered to the solder terminal. Then, a 4-wire configuration is used to finally prepare the measurement screw.
7. It’s absolutely recommended to ensure that the measurement screw is robust against mechanical impacts. Therefore, the paint-insulated copper wires should be shortened as much as possible to avoid any damage and minimize the temperature effects caused by the 2-wire cable. Further, the screw head should be protected (we used SG250 silicon). The following pictures illustrate examples of prepared measurement screws.
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